1. Field of the Invention
The present invention relates to optical isolators for use in optical communications fields in which semiconductor lasers and optical fibers are used, and in particular to polarizing glass constituting pigtail optical isolators.
2. Discussion of the Background
In optical communication in which a light source is realized by a semiconductor laser with a wavelength of 1.31 μm or 1.55 μm and a transmission line is realized by a silica-based fiber, an optical isolator is used to block feedback light, caused by reflection, traveling towards the light source to achieve a low error rate. The optical isolator typically includes a Faraday rotator, two polarizing elements, and a permanent magnet.
Polarizing glass including a glass substrate in which needle-like fine metal particles of silver or copper are dispersed such that the lengthwise directions thereof are oriented in a particular direction is generally used as the polarizing element for an optical isolator (hereinafter in this specification, this polarizing element is referred to as “polarizing glass containing dispersed fine metal particles”). The polarization effect in polarizing glass containing dispersed fine metal particles is attributable to the anisotropy of the plasmon resonance wavelength of the needle-like fine metal particles, and its polarizing characteristics are determined mainly by the aspect ratios of the needle-like fine metal particles (values obtained by dividing the lengthwise dimensions of the needle-like fine particles by the crosswise dimensions).
A method of manufacturing polarizing glass containing dispersed fine metal particles is described in detail in, for example, Japanese Unexamined Patent Application Publication No. 5-208844, and the manufacturing process is described briefly in the following.
<1> Glass materials including cuprous chloride are prepared with a desired composition, melted at a temperature of approximately 1450° C., and then cooled to room temperature.
<2> Thereafter, heat treatment is applied to cause precipitation of fine particles of cuprous chloride in the glass.
<3> After the fine particles of cuprous chloride are precipitated, a preform with an appropriate shape is produced by machining.
<4> The preform is stretched under predetermined conditions to obtain needle-like fine particles of cuprous chloride.
<5> Needle-like fine metal copper particles are obtained by reducing the stretched glass in a hydrogen atmosphere.
In polarizing glass containing dispersed fine metal particles that is manufactured by the above-described manufacturing process, the needle-like fine metal particles basically exist only in the vicinity of the surface layer of the glass, and the size of the region where they exist from the glass surface (hereinafter, the thickness from the surface is referred to as the “reduction layer thickness”) depends on the reduction conditions, such as the ambient temperature, exposure time to the reducing atmosphere, and so forth.
So-called free-space optical isolators have been generally used as optical isolators for optical communication.
FIG. 11 is a schematic cross-sectional side view of the optical system of a free-space optical isolator. In the figure, reference numerals 111 and 112 denote polarizing elements; reference numeral 113 denotes a Faraday rotator; reference numeral 114 denotes an optical isolator composed of the polarizing elements 111 and 112 and the Faraday rotator 113; reference numerals 115 and 115′ denote lenses; reference numeral 116 denotes an optical fiber; reference numeral 117 denotes a light source such as a semiconductor laser; and reference numerals 118 and 118′ denote a set of lines schematically showing beams of feedback light returning to the light source 117 (in particular, reference numeral 118′ denotes beams that have passed through the polarizing element 112). In the optical isolator 114 shown in FIG. 11, the polarization transmission axes of the polarizing elements 111 and 112 are arranged so as to form an angle of 45° relative to each other. Furthermore, regarding the Faraday rotator 113, its optical path length is set such that the polarization rotation angle is 45°. With the above-described arrangement, the beam (not shown in the figure) emitted from the light source 117 is converted into a collimated beam by the lens 115′, and only the light with polarization parallel to the polarization transmission axis of the polarizing element 112 is incident upon the Faraday rotator 113. The polarization direction of the light incident upon the Faraday rotator 113 is rotated by 45° through the Faraday effect of a permanent magnet (not shown in the figure). As described above, because the polarization transmission axes of the polarizing elements 111 and 112 make an angle of 45° relative to each other, the polarization direction of the light that has passed through the Faraday rotator 113 coincides with the polarization transmission axis of the polarizing element 111. Therefore, the light that has passed through the Faraday rotator 113 passes through the polarizing element 111 almost without loss, is converged by the lens 115, and then enters the optical fiber 116.
On the other hand, the feedback light beam 118 returning to the light source as a result of reflection at the optical fiber 116 or, for example, an optical element disposed downstream thereof (not shown in the figure) returns to the light source 117 in the opposite direction via the same optical path as that of the beam emitted from the above-described light source 117. In this case, because the polarization direction of the feedback light beam 118 that has passed through the Faraday rotator 113 forms an angle of 90° relative to the polarization transmission axis of the polarizing glass 112 (hereinafter, the axis differing by 90° relative to the polarization transmission axis is referred to as the “polarization extinction axis”) due to the irreciprocity of the Faraday rotator 113, its optical energy is greatly lost as it passes through the polarizing element 112.
In general, the performance of an optical isolator is evaluated based on the transmission loss of the light emitted from the light source and the isolation, which is the ability to block the feedback light 118. In particular, isolation is given by Expression (1) below, typically in units of decibels.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      ISO          ⁡                      (            dB            )                          =                              -            10                    ×                      log            ⁡                          (                                                P                                      88                    ′                                                                    P                  88                                            )                                                          (        1        )            
Here, ISO represents isolation, P88′ represents the power of the feedback light beam 118′, and P88 represents the power of the feedback light beam 118.
Isolation depends on the characteristics of the polarizing elements 111 and 112, variations of the rotation angle in the polarization direction in the Faraday rotator 113, and so forth. When the above-described known polarizing glass containing dispersed fine metal particles, in which needle-like fine particles of silver or copper are oriented and dispersed, is used as the polarizing elements 111 and 112, the isolation value is 30 dB or more, which is a level causing substantially no problems for practical use.
Nowadays, there is a demand for compact optical components, which has caused pigtail optical isolators to be commonly used. FIG. 14 is a schematic cross-sectional side view of the optical system of a pigtail optical isolator. In this drawing, reference numeral 141 denotes a needle-like fine metal particle included in the polarizing element 111; reference numeral 142 denotes arrows schematically showing the propagation directions of scattered light; and reference numeral 143 denotes an optical path of the feedback light beam.
The optical system of the pigtail optical isolator differs from the optical system of the free-space optical isolator shown in FIG. 11 in that (1) the optical fiber 116 is coupled directly to the polarizing element 111, and (2) only a single lens is provided. As a result, the optical path of the feedback light beam 143 differs from that of the feedback light beam 118, whereas the structures of the optical isolators 114 are almost same.
However, pigtail isolators have been problematic in that, when employing known polarizing glass containing dispersed fine metal particles in which needle-like fine particles of silver or copper are oriented and dispersed, in other words, polarizing glass that exhibits an isolation value of 30 dB or more when applied to a free-space optical isolator, the isolation value decreases to 23 to 27 dB, which is lower than the required specification of 30 dB.
As a result of intensive efforts to seek the cause of this problem, the inventors of the present invention have determined that the relevant cause lies in the fact that a pigtail optical isolator is more easily affected by scattered light in polarizing glass containing dispersed fine metal particles than a free-space optical isolator due to the difference in the optical systems of free-space optical isolators and pigtail optical isolators, revealing that it is necessary to decrease this scattered light to achieve the desired isolation level in a pigtail optical isolator.
Decreasing the volume of fine metal particles included in polarizing glass containing dispersed fine metal particles is effective to decrease scattered light, which will be described later in detail.
It should be noted, however, that the volumes of fine metal particles need to be decreased while maintaining certain aspect ratios because the polarizing characteristics of polarizing glass containing dispersed fine metal particles are determined by the aspect ratio of the substantially needle-like fine metal particles included in the glass.
For this purpose, metal halide fine particles with small volumes are precipitated in a glass substrate and then need to be stretched more intensely in the lengthwise direction in the subsequent stretching process to maintain their aspect ratios.
Not only are extensive facilities required in order to more intensely stretch the metal halide fine particles in the stretching process, but also glass is more likely to break in the stretching process, thereby decreasing the yield. Thus, this method cannot be considered a suitable manufacturing method.